A. Principles of Mechanical Ventilation
In many clinical settings mechanical ventilators are used to facilitate the respiratory flow of gas into and out of the lungs of patients who are sick, injured or anesthetized.
In general, mechanical ventilators provide a repetitive cycling of ventilatory flow, each such repetitive cycle being separated into two phases—an inspiratory chase followed by an expiratory phase.
The inspiratory phase of the ventilator cycle is characterized by the movement of positive-pressure inspiratory flow of gas through the ventilator circuit and into the lungs of the patient. The expiratory phase of the ventilatory cycle is characterized by cessation of the positive pressure inspiratory flow long enough to allow lung deflation to occur. The exhaled gas is vented from the ventilator circuit, typically through an exhalation valve. In patient whose lungs and thoracic musculature exhibit normal compliance, the act of exhalation is usually permitted to occur spontaneously without mechanical assistance from the ventilator.
It is sometimes desirable to control the airway pressure during exhalation to maintain a predetermined amount of positive back pressure during all, or a portion of, the respiratory cycle. Such techniques are often utilized to treat impairments of lung capacity due to pulmonary atelectasis or other factors.
The mechanical ventilators of the prior art have been grouped under various classification schemes, based on various criteria. In general, mechanical ventilators may be grouped or classified according to the parameter(s) which are utilized for a) triggering, b) limiting and c) terminating (e.g., cycling) the inspiratory phase of the ventilator cycle.
“Triggering” is the action that initiates the inspiratory phase of the ventilator cycle. The initiation of the inspiratory phase may be triggered by the ventilator or the patient. The variables and/or parameters which are utilized to trigger the beginning of the inspiratory phase include: time (i.e., respiratory rate), the commencement of spontaneous inhalation by the patient and/or combinations thereof.
“Limiting” of the inspiratory phase refers to the manner in which the inspiratory gas flow is maintained within prescribed ranges to optimize the ventilation of the patient's lungs. The limiting variables and/or parameters are typically controlled by the ventilator, but may change as a result of patient effort and/or physiologic variables such as lung compliance and airway resistance. The variables and/or parameters which are utilized for limiting the inspiratory phase include flow rate, airway pressure and delivered volume.
“Terminating” or “cycling” of the inspiratory phase of the ventilator cycle refers to the point at which the inspiratory flow is stopped and the ventilator and/or patient are permitted to “cycle” into the expiratory phase. Depending on the ventilator control settings, the termination of the inspiratory phase may be brought about by the ventilator or the patient. The variables and/or parameters which are utilized to terminate the inspiratory phase include: time; peak airway pressure; and/or tidal volume (Vt).
B. Mechanical Ventilation Modes Utilized in Modern Clinical Practice
In addition Mechanical ventilators are utilized to deliver various “modes” of mechanical ventilation, the particular mode of ventilation being selected or prescribed based on the clinical condition of the patient and the overall objective (i.e., long term ventilation, short term ventilation, weaning from ventilator, etc . . . ) of the mechanical ventilation.
I. Ventilation Modes:
i. Intermittent Mandatory Ventilation (IMV)                Intermittent Mandatory Ventilation is a ventilation mode wherein a spontaneously breathing patient receives intermittent mechanical inflation supplied asynchronously by the ventilator.        
ii. Synchronized Intermittent Mandatory Ventilation (SMIV)                Synchronized Intermittent Mandatory Ventilation is a ventilation mode wherein a spontaneously breathing patient receives occasional mandatory ventilatory breaths. Mandatory ventilator breaths are synchronized with the patient's spontaneous inspiratory efforts.        
iii. Controlled Mechanical Ventilation (CMV)                Controlled Mechanical Ventilation (CMV) is a ventilation mode wherein mechanical breaths are delivered to the patient at time intervals which are unaffected by patient efforts. Controlled Mechanical Ventilation is typically utilized in patients who are not breathing spontaneously.        
iv. Assist/Control Ventilation (A/C)                Assist/Control Ventilation (A/C) is a ventilation mode wherein the patient is able to volitionally alter the frequency of mandatory ventilator breaths received, but can not alter the flow and title volume (Vt) of each ventilator breath received. Controlled, mandatory breaths are initiated by the ventilator based on the set breath rate. In addition, the patient can demand and trigger an assist breath. After successful triggering of an assist breath, the exhalation valve is closed and gas is delivered to the patient to satisfy the preset tidal volume, peak flow and wave form.C. Breath Types Utilized in Modern Clinical Practice        
Breath types are typically classified according to the particular functions which control:                a) triggering;        b) limiting; and        c) cycling of each breath delivered by the mechanical ventilator, as described and defined hereabove.        
Typical breath types and ventilator parameters utilized in modern clinical practice include the following:
i. Machine-Cycled—Mandatory Breath                A machine-cycled, mandatory breath is a breath that is triggered, limited and cycled by the ventilator.        
ii. Machine-Cycled—Assist Breath                A machine cycled assist breath is a breath that is triggered by the patient, but is limited and cycled by the ventilator.        
iii. Patient-Cycled—Supported Breath                A patient-cycled, supported breath is a breath that is triggered by the patient, limited by the ventilator, and cycled by the patient.        
iv. Patient-Cycled—Spontaneous Breath                A patient-cycled spontaneous breath is a breath that is triggered, limited and cycled by the patient. While patient effort limits the flow, and hence the inspiratory volume of the breath, the ventilator may also limit the breath by providing a flow that is to low to maintain a constant pressure in the face of patient inspiratory demand.        
V. Volume-Controlled Mandatory Breaths                Volume-controlled breaths are machine-triggered mandatory breaths. The inspiratory phase is initiated by the ventilatory based on a preset breath rate. The inspiratory phase is ended, and the expiratory phase begun, when the breath delivery is determined to be complete based on a preset tidal volume, peak flow and wave form setting. The ventilator remains in expiratory phase until the next inspiratory phase begins.        
vi. Volume-Controlled—Assist Breaths                Volume-controlled breaths are machine cycled supported breaths that are initiated by the patient. Volume-controlled assist breaths may be initiated only when the “assist window” is open. The “assist window” is the interval or time during which the ventilator is programmed to monitor inspiratory flow for the purpose of detecting patient inspiratory effort. When a ventilator breath is triggered, the inspiratory phase of such breath will continue until a preset tidal volume peak flow and wave form have been achieved. Thereafter, the exhalation valve is open to permit the expiratory phase to occur. The ventilatory remains in the expiratory phase until the next patient-triggered breath, or the next mandatory inspiratory phase, begins.        
vii. Pressure-Controlled Breaths                Pressure-Controlled breaths are delivered by the ventilator using pressure as the key variable for limiting of the inspiratory phase. During pressure control, both the target pressure and the inspiratory time are set, and the tidal volume delivered by the ventilator is a function of these pressure and time settings. The actual tidal volume delivered in each pressure-controlled breath is strongly influenced by patient physiology.        
viii. Pressure Support Breaths                Pressure support breaths are triggered by the patient, limited by the ventilator, and cycled by the patient. Thus, each breath is triggered by patient inspiratory effort, but once such triggering occurs the ventilator will assure that a predetermined airway pressure is maintained through the inspiratory phase. The inspiratory phase ends, and the expiratory phase commences, when the patients inspiratory flow has diminished to a preset baseline level.        
ix. Sigh Breaths                A sigh breath is a machine-triggered and cycled, volume-controlled, mandatory breath, typically equal to 1.5 times the current tidal volume setting. The inspiratory phase of each sigh breath delivers a preset tidal volume and peak flow. The duration of the inspiratory phase of each sigh breath is limited to a maximum time period, typically 5.5 seconds. The ventilator may be set to deliver a sigh function automatically after a certain number of breaths or a certain time interval (typically 100 breaths for every 7 minutes), which ever interval is shorter. The sigh breath function it may be utilized during control, assist and SIMV modes of operation, and is typically disabled or not utilized in conjunction with pressure controlled breath types or continuous positive air way pressure (CPAP).        
x. Proportional Assist Ventilation (PAV)                Proportional Assist Ventilation (PAV) is a type of ventilator breath wherein the ventilator simply amplifies the spontaneous inspiratory effort of the patient, while allowing the patient to remain in complete control of the tidal volume, time duration and flow pattern of each breath received.        
xi. Volume Assured Pressure Support (VAPS)                Volume Assured Pressure Support (VAPS) is a type of ventilator breath wherein breath initiation and delivery is similar to a pressure support breath. Additionally, the ventilator is programmed to ensure that a preselected tidal volume (Vt) is delivered during such spontaneously initiated breath.D. Oxygen Enrichment of the Inspiratory Flow        
It is sometimes desirable for mechanical ventilators to be equipped with an oxygen-air mixing apparatus for oxygen enrichment of the inspiratory flow. Normal room air has an oxygen content (FiO2) of 21%. In clinical practice, it is often times desirable to ventilate patients with oxygen FiO2 from 21% to 100%. Thus, it is desirable for mechanical ventilators to incorporate systems for blending specific amounts of oxygen with ambient air to provide a prescribed oxygen-enriched FiO2. Typically, volume-cycle ventilators which utilize a volume displacement apparatus have incorporated oxygen mixing mechanisms whereby compressed oxygen is combined with ambient air to produce the selected FiO2 as both gases are drawn into the displacement chamber during the expiratory phase of the ventilator cycle. Nonbellows-type volume-cycled ventilators have incorporated other air-oxygen blending systems for mixing the desired relative volumes of oxygen and air, and for delivering such oxygen-air mixture through the inspirations circuitry of the ventilator.
E. Regulation/Control of Expiratory Pressure
The prior art has included separately controllable exhalation valves which may be preset to exert desired patterns or amounts of expiratory back pressure, when such back pressure is desired to prevent atelectasis or to otherwise improve the ventilation of the patient.
The following are examples of expiratory pressure modes which are frequently utilized in clinical practice:
i. Continuous Positive Airway Pressure (CPAP)                Continuous Positive Airway Pressure (CPAP) is employed during periods of spontaneous breathing by the patient. This mode of ventilation is characterized by the maintenance of a continuously positive airway pressure during both the inspiratory phase, and the expiratory phase, of the patient's spontaneous respiration cycle.        
ii. Positive End Expiratory Pressure (PEEP)                In Positive End Expiratory Pressure a predetermined level of positive pressure is maintained in the airway at the end of the expiratory phase of the cycle. Typically, this is accomplished by controlling the exhalation valve so that the exhalation valve may open only until the circuit pressure has decreased to a preselected positive level, at which point the expiration valve closes again to maintain the preselected positive end expiratory pressure (PEEP).F. Portable Ventilators of the Prior Art        
The prior art has included some non-complex portable ventilators which have inherent limitations as to the number and type of variables and/or parameters which may be utilized to trigger, limit and/or terminate the ventilator cycle. Although such non-complex ventilators of the prior art are often sufficiently power efficient and small enough for portable use, their functional limitations typically render them unsuitable for long term ventilation or delivery of complex ventilation modes and or breath types.
The prior art has also included non-portable, complex microprocessor controlled ventilators of the type commonly used in hospital intensive care units. Such ventilators typically incorporate a microcomputer controller which is capable of being programmed to utilize various different variables and/or parameters for triggering, limiting and terminating the inspiratory phase of the ventilator cycle. Complex ventilators of this type are typically capable of delivering many different ventilation modes and or breath types and are selectively operable in various volume-cycled, pressure cycled or time-cycled modes. However, these complex ventilators of the prior art have typically been too large in size, and too power inefficient, for battery-driven portable use. As a result of these factors, most of the complex micro-processor controlled ventilators of the prior art are feasible for use only in hospital critical care units.
As is well known there exist numerous settings, outside of hospital critical care units, where patients could benefit from the availability of a small, battery powered, complex microprocessor controlled mechanical ventilator capable of delivering extended modes of ventilation. For example, critically ill patients sometimes require transport outside of the hospital in various transport vehicles, such as ambulances and helicopters. Also, critical care patients are sometimes transiently moved, within the hospital, from the critical care unit to various special procedure areas (e.g., radiology department, emergency room, catheterization lab etc.,) where they may undergo diagnostic or therapeutic procedures not available in the critical care unit. Additionally, patients who require long term ventilation are not always candidates for admission to acute care hospital critical care units or may be discharged to step-down units or extended care facilities. Also, some non-hospitalized patients may require continuous or intermittent ventilatory support. Many of these patients could benefit from the use of complex microprocessor controlled ventilators, but may be unable to obtain such benefit due to the non-feasibility of employing such ventilators outside of the hospital-critical care unit environment.
In view of the foregoing limitations on the usability of prior art complex microprocessor controlled volume-cycled ventilators, there exists a substantial need in the art for the development of a portable, highly efficient, ventilator capable of programmed delivery of various modern ventilatory modes and breath types, while also being capable of use outside of the hospital critical care unit environment, such as in transport vehicles, extended care facilities and patients homes, etc.
U.S. Pat. No. 4,493,614 (Chu et al.) entitled “PUMP FOR A PORTABLE VENTILATOR” describes a reciprocating piston pump which is purportedly usable in a portable ventilator operable on only internal or external battery power.
U.S. Pat. No. 4,957,107 (Sipin) entitled “GAS DELIVERY MEANS” describes a rotating drag compressor gas delivery system which is ostensibly small enough to be utilized in a portable ventilator. The system described in U.S. Pat. No. 4,957,107 utilizes a high speed rotary compressor which delivers a substantially constant flow of compressed gas. The rotary compressor does not accelerate and decelerate at the beginning and end of each inspiratory phase of the ventilator cycle. Rather, the rotating compressor runs continuously, and a diverter valve is utilized to alternately direct the outflow of the compressor a) into the patients lungs during the inspiratory phase of the ventilation cycle, and b) through an exhaust pathway during the expiratory phase of the ventilation cycle.
Thus, there remains a substantial need for the development of an improved portable mechanical ventilator which incorporates the following features:                A. Capable of operating for extended periods (i.e., at least 2½ hours) using a single portable battery or battery pack as the sole power source;        B. Programmable for use in various different ventilatory modes, such as the above-described IMV, SMV, CMV, PAV, A/C and VPAS.        C. Usable to ventilate non-intubated mask patients as well as intubated patients.        D. Oxygen blending capability for delivering oxygen-enriched inspiratory flow.        E. Capable of providing controlled exhalation back pressure for CPAP or PEEP.        F. Portable, e.g., less than 30 lbs.        